Spindle motor

ABSTRACT

There is provided a means for preventing wear caused by the contacting/sliding of a thrust hydrodynamic gas bearing surface when a spindle motor is started. A cylinder  4  of a radial hydrodynamic gas bearing that has radial hydrodynamic grooves in an outer circumferential surface thereof and a disk  3  of a thrust hydrodynamic gas bearing that has thrust hydrodynamic grooves in an upper face thereof are disposed on an upper end of an axial center of a stator core  2  having a stator  2   a  around which a motor coil  7  is wound, a hollow cylinder  6  whose inner surface facing the cylinder  4  of the radial hydrodynamic gas bearing is smooth and a rotor magnet  8  facing the motor coil  7  are disposed on a hub  5  acting as a rotational member, a load in a radial direction is supported by the radial hydrodynamic gas bearing, and a load in a thrust direction is supported by using the thrust hydrodynamic gas bearing together with a magnetic bearing consisting of the stator  2   a  and the rotor magnet  8.

This application is a Divisional of Ser. No. 09/786,861 filed Mar. 12,2001 now U.S. Pat. No. 6,417,590.

TECHNICAL FIELD

The present invention relates to, in a rotating device in which aspindle motor provided with a hydrodynamic gas bearing that has a radialhydrodynamic gas bearing and a thrust hydrodynamic gas bearing acts as adriving source for a rotator, such as a magnetic disk, an optical disk,or a polygon mirror, a structure for preventing the wear of a thrusthydrodynamic gas bearing surface caused by contacting/sliding especiallywhen it starts.

BACKGROUND ART

In a rotating device for a rotator, such as a magnetic disk, an opticaldisk, or a polygon mirror, it is widely known that a spindle motorprovided with a hydrodynamic gas bearing is employed as a driving sourcefor the rotator. The reason is that the motor is characterized in, forexample, that the hydrodynamic gas bearing is simple in structure andcan be made more compact, that its noncontact rotation duringsteady-state rotation does not generate vibrations or rotationalirregularity that is caused by the bearing, that it is superior inhigh-speed durability, and that there is no contamination caused by thedispersion of a lubricant because oil, grease, or the like, is not used.

However, the disadvantage of the spindle motor provided with thehydrodynamic gas bearing is that the hydrodynamic surface of the thrustbearing is in contact when stopped, which causes wear of the surfacethereof due to contacting/sliding at the onset of operation. In order toovercome this disadvantage, there is a means in which the hydrodynamicsurface of the thrust bearing is floated when stopped, a thrust load isthen received by an axial center part of a cylinder of the radialhydrodynamic gas bearing that is a fixed member, and the axial centerpart of the cylinder is spaced by the thrust of radial hydrodynamicgrooves generated with the increase of the revolution speed of thespindle motor so as to maintain the gap of the thrust bearing to have aset value.

Its embodiment is proposed in Japanese Unexamined Patent Publication No.69715 of 1999. FIG. 10 shows the structure of a shaft fixing typespindle motor 100 therein. 101 is a base plate of a stator 110, 102 is acylindrical member used also as a shaft erected on the base plate 101,and 103 is a hollow cylindrical member that has a closed end. The hollowcylindrical member 103, the closed end of which is placed upward, isrotatably fitted onto the cylindrical member 102. A donut-shaped thrustmember 104 is integrally formed on the outer periphery of the hollowcylindrical member 103, and, at a position opposite to this, a thrustpressure member 106 is disposed through a cover 105 that engages withthe base plate 101. A rotor 108 acting as a rotator is fixed to a hub107 formed integrally with the hollow cylindrical member 103. A rotormagnet 109 is disposed on the outer circumferential surface of the lowerpart of the hollow cylindrical member 103, and, at a position oppositeto this, a motor coil 111 wound around the stator 110 that extends fromthe base plate 101 is disposed.

When the spindle motor 100 is stopped, the closed end of the hollowcylindrical member 103 and the top of the cylindrical member 102 comeinto contact with each other by the weight of the rotor 108 includingthe hub 107, and a gap between the thrust member 104 and the thrustpressure member 106 is sufficiently secured. When an electric current ispassed through the motor coil 111, the hollow cylindrical member 103rotates clockwise, viewed from the side of the rotor 108. And, as itsrevolution speed increases, a thrust occurs at a herringbone groove 112formed largely in the upper part of the outer circumferential surface ofthe cylindrical member 102, whereby the closed end of the hollowcylindrical member 103 and the top of the cylindrical member 102 drawaway from each other. Simultaneously, by thrust hydrodynamic that isgenerated by a spiral groove (not shown) formed in the upper face of thethrust member 104, a gap to the thrust pressure member 106 is reducedand the rotor 108 floats to a position where the thrust and the thrusthydrodynamic preserve a balance.

The publication states that, by the construction as described above, thethrust member 104 and the thrust pressure member 106 are prevented fromcontact and sliding during the steady-state rotation, and rises no wearwhatsoever occurs in this part. Additionally, it says that the rotor 108up by the thrust generated in the herringbone groove 112 of the radialhydrodynamic gas bearing, and therefore it is possible to obtain acompact spindle motor in which extra additional means other than thehydrodynamic gas bearing are omitted.

However, in the structure of the spindle motor of FIG. 10, since thethrust member 104 disposed on the outer periphery of the hollowcylindrical member 103 must undergo processing to form a spiral groove,the shape becomes complex, and an integral construction is uneconomical.Additionally, since the thrust pressure member 106 is situated above thethrust member 104 which floats upward, the base plate 101 and the cover105 are required to undergo processing for centering, thus making theshape complex and the assembly difficult. Furthermore, because of theaccumulative errors of these interrelated members, it is extremelydifficult to maintain the gap of the thrust hydrodynamic gas bearing tobe several-microns in order. Therefore, in order to solve theaforementioned problem, the present invention provides a spindle motorcapable of preventing contact between a fixed member and a rotatingmember also when stopped.

DISCLOSURE OF THE INVENTION

In a first embodiment, a cylinder of a radial hydrodynamic gas bearingthat has radial hydrodynamic grooves in an outer circumferential surfacethereof and a disk of a thrust hydrodynamic gas bearing that has thrusthydrodynamic grooves in an upper face thereof are disposed on an upperend on an axial center of a stator core having a stator around which amotor coil is wound; a hollow cylinder whose inner surface facing thecylinder of the radial hydrodynamic gas bearing is smooth and a rotormagnet facing the motor coil are disposed on a hub acting as arotational member; a load in a radial direction is supported by theradial hydrodynamic gas bearing; and a load in a thrust direction issupported by using the thrust hydrodynamic gas bearing together with amagnetic bearing consisting of the stator and the rotor magnet.

In a second embodiment, a hub acting as a rotational member is disposedon an upper end of a motor shaft provided with a rotor magnet on anouter periphery thereof; below the motor shaft, there are disposed adisk of a thrust hydrodynamic gas bearing that has thrust hydrodynamicgrooves in a lower face thereof and a cylinder of a radial hydrodynamicgas bearing that has radial hydrodynamic grooves in an outercircumferential surface thereof; as a fixed member, there are disposed ahollow cylinder whose inner surface facing the cylinder of the radialhydrodynamic gas bearing is smooth and a stator around which a motorcoil is wound, the stator facing the rotor magnet; a load in a radialdirection is supported by the radial hydrodynamic gas bearing; and aload in a thrust direction is supported by using the thrust hydrodynamicgas bearing together with a magnetic bearing consisting of the statorand the rotor magnet.

In a third embodiment, a cylinder of a radial hydrodynamic gas bearingthat has radial hydrodynamic grooves in an outer circumferential surfacethereof and a disk of a thrust hydrodynamic gas bearing that has thrusthydrodynamic grooves in an upper face thereof are disposed on an upperend on an axial center of a stator core having a stator around which amotor coil is wound; a hollow cylinder whose inner surface facing thecylinder of the radial hydrodynamic gas bearing is smooth and a rotormagnet facing the motor coil are disposed on a hub acting as arotational member; a secondary magnetic bearing is disposed thatcomprises a first permanent magnet shaped like a ring, the firstpermanent magnet fixed to an upper end surface of the cylinder, and asecond permanent magnet shaped like a ring, the second permanent magnetfixed to an upper end surface of the hollow cylinder in such a way as tosurround the first permanent magnet; a load in a radial direction issupported by the radial hydrodynamic gas bearing; and a load in a thrustdirection is supported by using together the thrust hydrodynamic gasbearing, the secondary magnetic bearing, and a primary magnetic bearingconsisting of the stator and the rotor magnet.

In a fourth embodiment, a hub acting as a rotational member is disposedon an upper end of a motor shaft provided with a rotor magnet on anouter periphery thereof; below the motor shaft, there are disposed adisk of a thrust hydrodynamic gas bearing that has thrust hydrodynamicgrooves in a lower face thereof and a cylinder of a radial hydrodynamicgas bearing that has radial hydrodynamic grooves in an outercircumferential surface thereof; in a case as a fixed member, there aredisposed a hollow cylinder whose inner surface facing the cylinder ofthe radial hydrodynamic gas bearing is smooth and a stator around whicha motor coil is wound, the stator facing the rotor magnet; a secondarymagnetic bearing that comprises a first permanent magnet shaped like aring is disposed, the first permanent magnet fixed to a lower endsurface of the cylinder, and a second permanent magnet shaped like aring, the second permanent magnet fixed to a lower end surface of thehollow cylinder in such a way as to surround the first permanent magnet;a load in a radial direction is supported by the radial hydrodynamic gasbearing; and a load in a thrust direction is supported by using togetherthe thrust hydrodynamic gas bearing, the secondary magnetic bearing, anda primary magnetic bearing consisting of the stator and the rotormagnet.

Preferably, the radial hydrodynamic grooves which exist in eachembodiment consist of at least three groove lines, each lead terminal ofwhich is formed in a range so as not to extend beyond the starting pointof an adjacent groove line in a development.

Additionally, even if the radial hydrodynamic groove that exists in eachembodiment is a herringbone groove having a groove length asymmetricalto the upper and lower parts, a similar effect is obtained.

And, it is preferable to use light, hard silicon nitride ceramics,silicon carbide ceramics, or alumina ceramics for members making up aradial hydrodynamic gas bearing and a thrust hydrodynamic gas bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1 b are sectional views of the first embodiment of thespindle motor of the present invention.

FIGS. 2a and 2 b are sectional views of the second embodiment of thespindle motor of the present invention.

FIGS. 3a and 3 b are sectional views of the third embodiment of thespindle motor of the present invention.

FIGS. 4a and 4 b are sectional views of the fourth embodiment of thespindle motor of the present invention.

FIG. 5 shows an example in which a herringbone groove is formed asthrust hydrodynamic grooves in the first embodiment of the presentinvention.

FIG. 6 shows an example in which herringbone grooves are formed asthrust hydrodynamic grooves in the third embodiment of the presentinvention.

FIG. 7 is an outline view of radial hydrodynamic grooves of the presentinvention.

FIG. 8 is a sectional view of a spindle motor of a first comparativeexample.

FIG. 9 is a sectional view of a spindle motor of a second comparativeexample.

FIG. 10 is a sectional view of a conventional spindle motor.

EMBODIMENT 1

Preferred embodiments in which the present invention is embodied will bedescribed in detail hereinafter with reference to the attached drawings.FIG. 1 shows the structure of a first embodiment in which a magneticbearing is constructed between a stator and a rotor magnet, wherein (a)is a diagram of a state when stopped, and (b) is a diagram ofsteady-state rotation.

In FIG. 1(a), reference character 1 designates a base plate for fixing astator core 2 as a fixed member. A stator 2 a that is made of a siliconsteel plate is disposed integrally with the outer periphery of thestator core 2, and is encircled with a motor coil 7. On the axial centerof the stator core 2, there are fixed a disk 3 of a thrust hydrodynamicgas bearing that has a spiral groove 3 a acting as thrust hydrodynamicgrooves and so forth in its upper face and a cylinder 4 of a radialhydrodynamic gas bearing that has spiral grooves 4 a acting as radialhydrodynamic grooves in its outer circumferential surface. And, a hollowcylinder 6 an inner surface of which is smooth is fixed at a positionwhere it faces the cylinder 4 inside a hub 5 acting as a rotationalmember. Furthermore, a rotor magnet 8 is disposed in the shape of a ringat a position where it faces a motor coil of a skirt 5 a of the hub 5.It is noted that a gap between the cylinder 4 and the hollow cylinder 6is set to have a difference of 5 μm or less in diameter. Rotators (notshown), such as a magnetic disk, an optical disk, and a polygon mirror,are mounted on the outer periphery of the hub 5.

The rotor magnet 8 and the stator 2 a constitute a magnetic bearing.When the spindle motor is stopped, the center location (ii) of the rotormagnet 8 should rest at the center location (i) of the stator 2 a thathas the strongest magnetic force, but, according to a balance betweenthe weight of the hub 5 and the magnetic force, it rests at a positionslightly descending from the center location (i), because no electriccurrent is passed through the motor coil 7. A clearance between theupper surface of the disk 3 that constitutes the thrust hydrodynamic gasbearing at this time and the bottom surface of the hollow cylinder 6becomes A1, and they are in the state of non-contact.

As shown in FIG. 1(b), when an electric current is passed through themotor coil 7 and thereby an alternating field is generated in the stator2 a, the hollow cylinder 6 rotates counterclockwise, viewed from theside of the hub 5. As the revolution speed increases, the hub 5 descendsbecause of the hydrodynamic of the hollow cylinder 6 that constitutesthe radial hydrodynamic gas bearing and the spiral grooves 4 a in theouter circumferential surface of the cylinder 4, so that a clearancebetween the upper surface of the disk 3 and the bottom surface of thehollow cylinder 6 becomes A2 as a result of the balance with repulsionthat occurs in the spiral grooves 3 a. Normally, the revolution speed ofthe hub 5 reaches 12000 to 18000 rpm, and A2 is maintained at theclearance of 2 to 3 μm. During steady-state rotation, since the hub 5descends because of the hydrodynamic of the spiral grooves 4 a, thecenter location (ii) of the rotor magnet 8 becomes lower than whenstopped, and is deviated from (i) by A3, thus slightly decreasing themotor efficiency.

FIG. 2 shows the structure of a second embodiment in which a magneticbearing is constructed between the stator 2 a and the rotor magnet 8 inthe same manner as in the first embodiment, with the side of the hollowcylinder 6 functioning as a fixed member. FIG. 2(a) is a diagram of astate when stopped, and FIG. 2(b) is a diagram of steady-state rotation.

In FIG. 2(a), reference character 9 designates a motor shaft, on theouter periphery of which the rotor magnet 8 is disposed, and a hub 5acting as a rotational member is fixed to the upper end. Below the motorshaft 9, there are fixed a disk 3 of a thrust hydrodynamic gas bearingthat has spiral grooves 3 a, etc., acting as thrust hydrodynamic groovesin its undersurface and a cylinder 4 of a radial hydrodynamic gasbearing that has spiral grooves 4 a acting as radial hydrodynamicgrooves in its outer circumferential surface. A hollow cylinder 6 aninner surface of which is smooth is fixed to a case 10 as a fixed memberat a position facing the cylinder 4, and a stator 2 a that is made of asilicon steel plate and is encircled with a motor coil 7 is disposedintegrally therewith at a position facing the rotor magnet 8.

When stopped, the undersurface of the disk 3 that constitutes the thrusthydrodynamic gas bearing and the upper end surface of the hollowcylinder 6 are mutually in a state of non-contact, and its clearance isA1. When an electric current is passed through the motor coil 7 andthereby an alternating field is generated in the stator 2 a, thecylinder 4 rotates counterclockwise, viewed from the side of the hub 5,as shown in FIG. 2(b), and the hub 5 descends. A clearance between theundersurface of the disk 3 and the upper end surface of the hollowcylinder 6 becomes A2 as a result of the balance with repulsiongenerated by the spiral grooves 3 a. Accordingly, the same operationaleffect as in the first embodiment is obtained. The center location (ii)of the rotor magnet 8 moves downward during steady-state rotation, anddeviates from the center location (i) of the motor coil 7 by A3, thusdecreasing the motor efficiency slightly, in the same manner as in thefirst embodiment.

FIG. 3 shows the structure of a third embodiment in which a ring-shapedpermanent magnet is fixed onto the concentric circle of the respectiveupper end surfaces of the cylinder 4 and the hollow cylinder 6 so as toconstruct a secondary magnetic bearing. FIG. 3(a) is a diagram of astate when stopped, and FIG. 3(b) is a diagram of steady-state rotation.

In FIG. 3(a), reference character 1 designates a base plate for fixing astator core 2 as a fixed member. A stator 2 a that is made of a siliconsteel plate is disposed integrally with the outer periphery of thestator core 2, and is encircled with a motor coil 7. On the axial centerof the stator core 2, there are fixed a disk 3 of a thrust hydrodynamicgas bearing that has a spiral groove 3 a acting as thrust hydrodynamicgrooves and so forth in its upper face and a cylinder 4 of a radialhydrodynamic gas bearing that has spiral grooves 4 a acting as radialhydrodynamic grooves in its outer circumferential surface. And, a hollowcylinder 6 an inner surface of which is smooth is fixed at a positionwhere it faces the cylinder 4 inside a hub 5 acting as a rotationalmember. Furthermore, a rotor magnet 8 is disposed in the shape of a ringat a position where it faces a motor coil of a skirt 5 a of the hub 5,thereby constructing a primary magnetic bearing, in exactly the samemanner as in the first embodiment.

In the first embodiment, the center location (iv) of the rotor magnet 8sinks below the center location (iii) of the stator 2 a because of theown weight of the hub 5 when stopped, but, in the third embodiment, inorder to prevent this, a first permanent magnet 11 shaped like a ring isfixed to the upper end surface of the cylinder 4, and a second permanentmagnet 12 shaped like a ring is fixed to the upper end surface of thehollow cylinder 6 in a manner so as to surround the first permanentmagnet 11, thereby constructing a secondary magnetic bearing. If theoutside of the first permanent magnet 11 is designed as a south poleand, on the other hand, the inside of the second permanent magnet 12 isdesigned as a north pole, or vice versa, the respective permanentmagnets 11 and 12 attract each other so as to support the weight of thehub 5, and the center location (iv) of the rotor magnet 8 rests at aposition higher than the center location (iii) of the stator 2 a. Theouter diameter of the first permanent magnet 11 is set at a slightlysmaller value than the outer diameter of the cylinder 4, and the innerdiameter of the second permanent magnet 12 is set at a slightly largervalue than the inner diameter of the hollow cylinder 6, and they arefixed, for example, by an anaerobic adhesive that is hardened byultraviolet rays. Accordingly, by enlarging a face-to-face gap betweenthe permanent magnets 11 and 12 to be more than a face-to-face gap ofthe radial hydrodynamic gas bearing, the cylinder 4 and the hollowcylinder 6 are brought into contact together before the contact betweenthe permanent magnets 11 and 12, thus facilitating the heatingprocessing of the adhesive also.

The clearance between the upper surface of the disk 3 and the bottomsurface of the hollow cylinder 6 when stopped in FIG. 3(a) becomes A1,and they are mutually maintained in the state of non-contact. When anelectric current is passed through the motor coil 7, and the hollowcylinder 6 reaches the state of steady-state rotation in which thehollow cylinder 6 rotates counterclockwise viewed from the side of thehub 5, the hub 5 descends to the position of A2 because of thehydrodynamic in the spiral grooves 4 a in the outer circumferentialsurface of the cylinder 4 and the hollow cylinder 6 that constitute aradial hydrodynamic gas bearing. In this state, since the center (iv) ofthe rotor magnet 8 coincides with the center (iii) of the motor coil 7in the optimum zone of magnetic lines of force, the efficiency of themotor can be optimized.

FIG. 4 shows the structure of a fourth embodiment in which a secondarymagnetic bearing is constructed in the same manner as in the thirdembodiment, with the side of the hollow cylinder 6 functioning as afixed member. FIG. 4(a) is a diagram of a state when stopped, and FIG.4(b) is a diagram of steady-state rotation.

In FIG. 4(a), reference character 9 designates a motor shaft, on theouter periphery of which the rotor magnet 8 is disposed, and a hub 5acting as a rotational member is fixed to the upper end. Below the motorshaft 9, there are fixed a disk 3 of a thrust hydrodynamic gas bearingthat has spiral grooves 3 a, etc., acting as thrust hydrodynamic groovesin its undersurface and a cylinder 4 of a radial hydrodynamic gasbearing that has a spiral groove 4 a acting as radial hydrodynamicgrooves in its outer circumferential surface. A hollow cylinder 6 aninner surface of which is smooth is fixed to a case 10 as a fixed memberat a position facing the cylinder 4, and a stator 2 a that is made of asilicon steel plate and is encircled with a motor coil 7 is disposedintegrally therewith at a position facing the rotor magnet 8, therebyconstructing a primary magnetic bearing, in exactly the same manner asin the second embodiment.

Further, a first permanent magnet 11 shaped like a ring is fixed to thebottom surface of the cylinder 4, and a second permanent magnet 12shaped like a ring is fixed to the bottom surface of the hollow cylinder6 in such a way as to surround the first permanent magnet 11 so as toconstruct a secondary magnetic bearing, in exactly the same manner as inthe third embodiment.

The clearance between the disk 3 and the hollow cylinder 6, which are ina non-contact state, when stopped in FIG. 4(a) becomes A1, and when anelectric current is passed through the motor coil 7, and the hollowcylinder 6 reaches the state of steady-state rotation in which thehollow cylinder 6 rotates counterclockwise viewed from the side of thehub 5, the hub 5 descends to the position of A2 as shown in FIG. 4(b).In this state, since the center (iv) of the rotor magnet 8 coincideswith the center (iii) of the motor coil 7 in the optimum zone ofmagnetic lines of force, the efficiency of the motor can be optimized asin the third embodiment.

In any of the first to fourth embodiments, the radial hydrodynamicgrooves formed in the outer periphery of the cylinder 4 may beherringbone grooves asymmetrical about the upper and lower parts asshown in FIGS. 5 and 6. Additionally, grooves as shown in FIG. 7 isformed, i.e., at least three groove lines 4 a are formed in thecircumferential surface. And, it is preferable to form the terminal Y1of the groove line 4 a in a range so as not to extend beyond theadjacent starting point X2 of the groove line 4 a in a developed state.Additionally, from the starting point X1 to the terminal Y1, the grooveline 4 a may be a straight line or a spiral curve. The groove line 4 ais normally 1 to 3 mm in width, and it may be a shallow groove of aboutseveral micrometers to several tens of micrometers in depth, and,according to how a whetstone is applied, the hollow cylinder may beformed to have a shaved flat surface. In the aforementioned groove line4 a, the number of processing steps can be reduced more significantlythan with the herringbone groove, and an equal effect is achieved.

In any of the first to fourth embodiments, it is preferable to make atleast the disk 3, the cylinder 4, and the hollow cylinder 6 byceramic-made members, such as alumina, silicon nitride, and siliconcarbide. Among these materials, alumina is the most cost-effective.

Next, a description will be provided of the results of preparing andcomparing comparative examples 1 and 2 in order to confirm theoperational effect of embodiments 1 and 3. FIG. 8 shows the structureduring steady-state rotation of a spindle motor of comparative example 1compared with embodiment 1. The names, signs, and operations ofconstituent elements in FIG. 8 are exactly the same as in embodiment 1,and therefore their description is omitted. However, there is adifference in that, when the motor is stopped, the upper surface of thedisk 3 and the bottom surface of the hollow cylinder 6, which constitutea thrust hydrodynamic gas bearing, are brought into contact together.

FIG. 9 shows the structure during steady-state rotation of a spindlemotor of comparative example 2 compared with embodiment 3. The names,signs, and operations of constituent elements in FIG. 9 are exactly thesame as in embodiment 3, and therefore their description is omitted.However, the groove line 4 a formed in the outer periphery of thecylinder 4 may be a herringbone groove that is even about the upper andlower parts or may be a longitudinal groove that is parallel to an axialcenter and straight. And, there is a difference in that the disk 3 thatconstitutes a thrust hydrodynamic gas bearing is omitted, and a thrustload is supported only by the primary and secondary magnetic bearings.

Table 1 shows results obtained such that the thrust load of embodiments1, 3 and comparative examples 1, 2 is set constantly at 150 g, thespindle motor, the specification of which is a dc 12V rating, is drivenat a steady-state revolving speed of 18000 rpm, and the up-and-downmotion of the upper surface of the hub 5 is observed with a laserdisplacement gauge.

TABLE 1 Current value Up-and-down motion of hub Sample at start duringsteady-state rotation embodiment 1 1.2 A 1 μm or less embodiment 3 1.2 A1 μm or less comparative ex. 1 2.8 A 1 μm or less comparative ex. 2 1.2A max 30 μm

The results in Table 1 show that the up-and-down motion of the hub 5during steady-state rotation is 1 μm or less in embodiments 1, 3, andcomparative example 1, and the magnetic head of a reader is in a rangein which it can follow the surface of the rotator of the magnetic disk.Therefore, the rotator never collide with the magnetic head. However, incomparative example 2, a thrust hydrodynamic gas bearing is omitted, anda thrust load is supported only by a magnetic bearing, and therefore thehub 5 floats unstably, which is unpractical.

In comparative example 1, as a current for starting, approximately 2.3times that of embodiments 1 and 3 and comparative example 2 is requiredfor generating rotation torque conquering the initial sliding resistancesince the upper surface of the disk 3 and the lower end surface of thehollow cylinder 6 contact together in the stop condition. Concerningembodiment 1, since the best zone of magnetic lines of force of therotor magnet 8 deviates slightly from the center of the motor coil 7during steady-state rotation, the motor efficiency decreases, and acurrent value has a slight tendency to increase, but it is in apractically negligible range.

INDUSTRIAL APPLICABILITY

When the motor is stopped, the thrust load of a hub is supported by aprimary magnetic bearing made up of a stator and a rotor magnet or incombination with a cylinder and a secondary magnetic bearing made up ofa pair of ring-shaped permanent magnets disposed on the end face of ahollow cylinder, and the disk of the thrust hydrodynamic gas bearing andthe end face of the hollow cylinder are maintained in a non-contactstate. When the rotor magnet is activated, a force reducing theclearance of the thrust hydrodynamic gas bearing is generated by theradial hydrodynamic grooves of the radial hydrodynamic gas bearing sothat the disk of the thrust hydrodynamic gas bearing and the hollowcylinder approach each other, and, during steady-state rotation, sincethe thrust load is supported chiefly by the repulsion of the thrusthydrodynamic gas bearing, it is possible to provide a spindle motor inwhich the constituent elements of the thrust hydrodynamic gas bearingare consistently prevented from coming in to contact together, and thereis no need for concern over the wear of the elements. Also less electricpower consumption is used at the start time. And, as is clear from FIGS.1 to 4, since it has a structure that facilitates the centering of theinterrelated members, it is also easy to maintain the gap of the thrusthydrodynamic gas bearing to have a value of several micrometers.

What is claimed is:
 1. A spindle motor characterized in that itcomprises: a cylinder of a radial hydrodynamic gas bearing having radialhydrodynamic grooves formed in an outer circumferential surface thereof;a disk of a thrust hydrodynamic gas bearing having thrust hydrodynamicgrooves formed in an upper face thereof, the cylinder and the disk beingdisposed on and supported by an upper end of a stator core on an axialcenter of the stator core, the stator cow having a stator around which amotor coil is wound; a hollow cylinder whose inner surface facing thecylinder of the radial hydrodynamic gas bearing is smooth; and a rotormagnet facing the motor coil, the hollow cylinder and the rotor magnetbeing disposed on a hub acting as a rotational member; wherein a load ina radial direction is supported by the radial hydrodynamic gas bearing,and a load in a thrust direction is supported by using the thrusthydrodynamic gas bearing together with a magnetic bearing consisting ofthe stator and the rotor magnet.
 2. A spindle motor according to claim1, wherein the radial hydrodynamic grooves in the outer periphery of thecylinder consist of at least three groove lines each of the groovesextending along at least substantially all of an entire length of thecylinder, and, in a development, a terminal of a lead of the groove lineis formed in a range so as not to extend beyond a starting point of anadjacent groove line.
 3. A spindle motor according to claim 1, whereinthe radial hydrodynamic grooves in the outer periphery of the cylinderare herringbone grooves comprising a first leg longer than a second legthereof so as to form a groove length asymmetrical with respect to theupper and lower pans of the cylinder.
 4. A spindle motor according toclaim 1, wherein, when stopped, a thrust load of the hub is supported bythe magnetic bearing, and the disk of the thrust hydrodynamic gasbearing and an end face of the hollow cylinder are maintained in anon-contact state, and, as the rotor magnet rotates, a force reducingthe clearance of the thrust hydrodynamic gas bearing works by the radialhydrodynamic grooves of the radial hydrodynamic gas bearing, so that thedisk of the thrust hydrodynamic gas bearing and the hollow cylinderapproach each other, and, during steady-state rotation, a thrust load issupported chiefly by repulsion of the thrust hydrodynamic gas bearing.5. A spindle motor according to claim 1, wherein ceramics are used for amember that constitutes the radial hydrodynamic gas bearing and thethrust hydrodynamic gas bearing.
 6. A spindle motor characterized inthat it comprises: a hub acting as a rotational member disposed on anupper end of a motor shaft provided with a rotor magnet on an outerperiphery thereof, a disk of a thrust hydrodynamic gas bearing havingthrust hydrodynamic grooves formed in a lower face thereof, a cylinderof a radial hydrodynamic gas bearing having radial hydrodynamic groovesformed in an outer circumferential surface thereof, the disk and thecylinder being disposed directly below and adjacent the motor shaft, ahollow cylinder having a smooth inner surface positioned to face thecylinder of the radial hydrodynamic gas bearing and having a top surfacepositioned to face the disk thrust hydrodynamic grooves, and a statoraround which the motor coil is wound, the stator facing the rotormagnet, the hollow cylinder and the stator being disposed in a case as afixed member, wherein a load in a radial direction is supported by theradial hydrodynamic gas bearing, and a load in a thrust direction issupported by using the thrust hydrodynamic gas bearing together with amagnetic bearing consisting of the stator and the rotor magnet.
 7. Aspindle motor comprising, in combination: a cylinder of a radialhydrodynamic gas bearing having radial hydrodynamic grooves formed in anouter circumferential surface thereof; a disk of a thrust hydrodynamicgas bearing having thrust hydrodynamic grooves formed in an upper facethereat the cylinder and the disk being disposed on an upper end on anaxial center of a stator core having a stator around which a motor coilis wound; a hollow cylinder having a smooth inner surface positioned onan inner surface of a hub acting as a rotational member to face thecylinder of the radial hydrodynamic gas bearing; and a rotor magnetdisposed on said hub to face the stator motor coil, wherein a load in aradial direction is supported by the radial hydrodynamic gas bearing,wherein a load in a thrust direction is supported by using the thrusthydrodynamic gas bearing together with a magnetic bearing consisting ofthe stator and the rotor magnet, and wherein said rotor magnet and saidmotor coil are configured, in combination, to magnetically balance aweight of at least said hub and said hollow cylinder and said rotormagnet attached thereto so as to maintain a first clearance between saiddisk thrust hydrodynamic grooves and a bottom surface of said hollowcylinder in a static state and to maintain a second clearance betweensaid disk thrust hydrodynamic grooves and said bottom surface of saidhollow cylinder in a dynamic steady-state condition.
 8. A spindle motoraccording to claim 7, wherein said first clearance between said diskthrust hydrodynamic grooves and said bottom surface of said hollowcylinder is larger than said second clearance between said disk thrusthydrodynamic grooves and said bottom surface of said hollow cylinder. 9.A spindle motor comprising, in combination: a hub acting as a rotationalmember disposed on an upper end of a motor shaft provided with a rotormagnet disposed about an outer circumferential surface thereof; a diskof a trust hydrodynamic gas bearing having thrust hydrodynamic groovesformed in a lower face thereof, a cylinder of a radial hydrodynamic gasbearing having radial hydrodynamic grooves formed in an outercircumferential surface thereof, the disk and the cylinder beingdisposed below the motor shaft in a fixed relation thereto, a hollowcylinder having a smooth inner surface positioned to face the cylinderof the radial hydrodynamic gas bearing and positioned so that a topsurface of the hollow cylinder faces the disk thrust hydrodynamicgrooves, and a stator around which the motor coil is wound, the statorfacing the rotor magnet, the hollow cylinder and the stator beingdisposed in a case as a fixed member, wherein a load in a radialdirection is supported by the radial hydrodynamic gas bearing, andwherein a load in a thrust direction is supported by using the thrusthydrodynamic gas bearing together with a magnetic bearing consisting ofthe stator and the rotor magnet, wherein said rotor magnet and saidmotor coil are configured, in combination, to magnetically balance aweight of at least said hub and said motor shaft, said disk, and saidcylinder attached thereto so as to maintain a first clearance betweensaid disk thrust hydrodynamic grooves and a top surface of said hollowcylinder in a static state and to maintain a second clearance betweensaid disk thrust hydrodynamic grooves and said top surface of saidhollow cylinder in a dynamic steady-state condition.
 10. A spindle motoraccording to claim 9, wherein said first clearance between said diskthrust hydrodynamic grooves and said top surface of said hollow cylinderis larger than said second clearance between said disk thrusthydrodynamic grooves and said top surface of said hollow cylinder.